1. Field of the Invention
Exemplary embodiments of the present invention relate to a light emitting device, and more particularly, to an AC light emitting device in which a long-persistent phosphor is adopted to prevent a flicker effect and a light emitting device module having the same.
2. Discussion of the Background
Recently, a light emitting device that is directly connected to an alternating current (AC) power source to operate under AC power has been developed. Such a light emitting device includes a light emitting diode chip having a plurality of light emitting cells on a single substrate. The light emitting device may directly operate under AC power by connecting the arrays of light emitting cells to each other in anti-parallel or by connecting the array of light emitting cells to a bridge rectifier.
However, whenever a phase of the AC power is changed, On/Off operations are reiterated in the light emitting diode chip that operates under AC power so that homogeneous light cannot be continuously emitted from the light emitting diode chip and a flicker effect may be generated. When the AC light emitting device is used for a long time for the purpose of lighting, such a flicker effect may cause the fatigue of human eyes. Meanwhile, when the AC light emitting device is installed to an object moving at a rapid speed such as a vehicle, the flicker effect may cause the object to be observed with the naked eye.
In order to prevent such a flicker effect, an AC light emitting device using a delay unit has been disclosed in Korean Laid-Open Patent Publication No. 10-2007-47056 while an AC light emitting device adopting a long-persistent phosphor has been disclosed in Korean Patent No. 10-746952. According to these references, the flicker effect may be alleviated either by delaying a phase of AC power waveform or by emitting another light from the long-persistent phosphor which has a long decay time when the phases of the AC power change.
However, if a circuit such as a delay unit is additionally mounted together with an AC light emitting device, the cost required to install and operate the AC light emitting device increases, so that the substitution effect of the light emitting device for the general illumination is reduced. Further, since the afterglow luminescence of the light emitted from the long-persistent phosphor is relatively smaller than the luminescence of the light emitted from the light emitting diode, the alleviation of the flicker effect using the long-persistent phosphor has a limitation. In order to further alleviate the flicker effect, the amount of the long-persistent phosphor may be increased. However, an increase in the usage amount of the long-persistent phosphor causes an increase in the loss of light emitted from the light emitting diode, so that the total light emission output may be reduced.